This invention relates to the art of forming fold lines in relatively rigid plastic sheet material such as polyolefins, and more particularly to a process and apparatus for forming fold lines in such material and to an improved container formed thereby.
With the recent development of extruding synthetic plastic materials, such as those in the polyolefin family, in which uniformity of sheet thickness and flatness are now attainable, it has now become a suitable material for the fabrication of folding cartons.
Previous efforts at forming fold lines in a sheet plastic carton blank to provide the necessary flexibility at the fold line to permit setting up the box from the flat blank have proved to be unsatisfactory due to the fact that damage or cracking of the sheet material occurs at or near the fold lines during folding. Also, when the box blank is flattened for storage or shipment, or when the flattened box is erected preparatory to filling, the panels and flaps of the box do not remain in the desired condition, but rather tend to revert to a state intermediate the flattened and opened conditions, due to the springness or elastic recovery tendency of the material being used to form the box. This elastic behavior tends to interfere with the automatic folding, filling and closing operations currently being practiced in commercial packaging facilities using paperboard boxes and cartons.
Scoring of sheet material to form a fold line has heretofore been performed by bringing a scoring blade against the sheet material, which is backed up by a platen, so that the sheet material is deformed out of the plane of the sheet to form the desired score line. In forming conventional paperboard boxes and cartons, the blades are unheated, and the fold line is formed by what is referred to as "cold scoring," as described for example in U.S. Pat. No. 3,137,217 and U.S. Pat. No. 3,529,516. Similar cold scoring techniques have been attempted in connection with plastic sheeting as indicated, i.e., in U.S. Pat. No. 3,334,802; U.S. Pat. No. 3,292,513; U.S. Pat. No. 3,594,464 and U.S. Pat. No. 3,350,492. Unfortunately, cold scoring has heretofore proven unsatisfactory when used in conjunction with existing thermoplastic sheet materials due to the fact that the cold deformation of these sheet materials, along the intended fold line, appears to produce a strain hardening of the material, resulting in fracturing and crazing along the score line, when the sheet material is folded or set-up into the carton. This problem is recognized in U.S. Pat. No. 3,589,022, which describes attempts to obviate this difficulty by employing heated scoring blades to form the desired score line indentation on one side of the sheet. It has, however, been found that even where the scoring blade is heated and brought down on one side of the sheet material sandwiched between the scoring blade and an unheated platen, with the scoring blade penetrating the sheet material on one side of the sheet, there is still produced a weakness and crazing discoloration of the plastic material, though perhaps not as great as in connection with cold scoring.
Other methods of hot-scoring plastic sheet material are described in U.S. Pat. No. 3,379,814. The use of plastic molding techniques for imparting fold lines is described in U.S. Pat. Nos. 3,132,649 and 3,201,145.
Therefore, a need has existed in the packaging field for a plastic material suitable for use in manufacturing folding cartons and boxes that would work in the identical way paperboard boxes do, and in existing folding carton making machinery, without the drawbacks of the prior art, which would be durable, easily handled and decorated in the same manner as folding paperboard containers.